1. Field of the Invention
The present invention is generally in the field of semiconductor fabrication. More particularly, the invention is in the field of fabricating emitter structures in transistors.
2. Background Art
In silicon-germanium (xe2x80x9cSiGexe2x80x9d) heterojunction bipolar transistor (xe2x80x9cHBTxe2x80x9d) technology, one approach to formation of an emitter in the SiGe HBT includes filling an emitter window opening with polysilicon material. One conventional method of filling the emitter window opening with polysilicon material comprises depositing undoped polysilicon into the emitter window opening, implant doping the polysilicon, and activating the dopants in the polysilicon.
Disadvantageously, the aforementioned method for fabricating SiGe HBTs can produce transistors having different characteristics depending on emitter window widths. Polysilicon deposits relatively conformally within an emitter window opening having a wide emitter window width. However, an unwanted xe2x80x9cplug effectxe2x80x9d can occur for emitters having narrow emitter window widths. The plug effect refers to non-conformal deposition of undoped polysilicon within a narrow emitter window, resulting in a higher thickness of polysilicon in the center of the emitter window relative to the emitter window sidewalls; hence resembling a xe2x80x9cplugxe2x80x9d in the narrow emitter window. The plug effect, combined with implant doping of the emitter, also result in a difference in the doping profile in the emitter and base of transistors having narrow emitter windows relative to transistors having wide emitter windows.
Another disadvantage of the conventional methods occurs due to the implant doped polysilicon being in direct contact with the base. Activating dopants in the implant doped polysilicon produces a difficult-to-control diffusion into the base, which degrades HBT performance.
Therefore, a need exists for fabricating transistors, such as SiGe HBTs, having substantially similar characteristics regardless of the emitter window width, and for controlled diffusion of dopants into the base, to achieve improved performance.
The present invention is directed to method for fabrication of emitter of a transistor and related structure. The invention resolves the need in the art for fabricating transistors, such as SiGe HBTs, having substantially similar characteristics regardless of the emitter window width, and for controlled diffusion of dopants into the base, thus achieving improved performance.
According to one embodiment, the present invention is a method for fabricating an emitter structure, comprising a step of conformally depositing an undoped polysilicon layer in an emitter window opening and over a base. Next, a doped polysilicon layer is non-conformally deposited over the undoped layer. Thereafter, the steps of conformally depositing an undoped polysilicon layer and non-conformally depositing a doped polysilicon layer are repeated until the emitter window opening is filled. The method can further comprise a step of activating dopants. In other embodiments, the present invention is a structure fabricated according to the above method. Other features and advantages of the present invention will become more readily apparent to those of ordinary skill in the art after reviewing the following description and accompanying drawings.